The present invention relates to a method and apparatus for correcting problems associated with drift occurring in a cathode ray tube (CRT) and, more particularly, to prevent changes of intensity due to factors such as aging and temperature variations so as to permit accurate and consistently excellent quality from high resolution color hard copy printers which utilize CRT's as an image source.
It has long been known that changes in temperature which naturally occur in a CRT during use can cause dimensional changes between its electrodes and thus change the intensity or brightness of the image on the CRT face. Similar effects can occur as the CRT age due to phosphors deteriorating and reductions in cathode emission. The change in intensity from these effects is sometimes referred to as drift and, for normal television viewing, are generally unnoticed by a viewer.
In some applications, however, it is necessary for the intensity of radiation on the face of the cathode ray tube to remain constant for a given input and effects of drift have heretofore been objectionable. For example, in the art of graphic recording, a light sensitive paper is caused to move across the face of a cathode ray tube to provide a permanent record of the image produced by the CRT. Most writing media such as dry silver paper are quite sensitive to light intensity and, particularly when writing low density colors, the effects of drift are quite noticeable. Accordingly, it is desirable to eliminate the effects of drift in such systems so to assure consistently high quality reproductions.
Attempts to control the intensity of illumination on the face of the CRT have been made in the past by measuring the brightness of the CRT face and changing the bias of the cathode in such a way that the brightness remains constant. Unfortunately, adjusting the cathode bias undesirably causes the grid modulation gain to change.